M1 macrophages, also known as classically activated macrophages, are a pro-inflammatory phenotype that play a crucial role in host defense by producing cytokines like IL-1, IL-6, and TNF-α. They are typically induced by microbial products and Th1 cytokines such as IFN-γ, and are involved in the destruction of pathogens and tumor cells, as well as promoting inflammation and tissue damage in chronic inflammatory diseases.